This invention relates generally to ion trap mass spectrometers, and more particularly the invention relates to ion focusing and gating optics for use in an ion trap mass spectrometer.
Atmospheric pressure ionization sources are coupled to mass spectrometers with ion optic assemblies. Ions formed in an atmospheric pressure ionization source are passed through a sampling orifice into a vacuum chamber and then through a beam skimmer into a quadrupole, hexapole or octopole ion guide and transported to an exit lens having a small aperture. Ions passing through the exit lens pass through a second ion optics stage including an ion guide focus lens and end cap lens which cooperatively function to direct the ion beam into the entrance hole of the ion trap end cap. The ion trap selects ions for passage on to an ion detector.
Electrospray ionization occurs at atmospheric pressure and produces charged droplets having a wide distribution of droplet diameters. The largest of these droplets are removed by applying a flow of heated gas that flows away from the entrance orifice into the vacuum chamber. When the droplets are large, the gas flow is sufficient to transport the droplets away from the orifice against the attractive force of an electric field existing between the spray source and the entrance orifice. Droplets that are very small are evaporated by the heated gas to produce charged sample ions that are easily attracted by the electric field to the entrance orifice, moving counter to the flow of gas. Droplets of intermediate size are also attracted by the electric field into the orifice and thereby into the vacuum chamber. The ions exit the orifice, or vacuum restriction that limits the gas load on the vacuum pumps, moving almost at the velocity of the expanding gas, which is supersonic. Hence the kinetic energy of droplets moving at nearly the same velocity as the gas will have very high kinetic energies due to the large mass of the droplets. It is undesirable to allow these charged droplets to enter the trap when the trapped ions are being scanned out for detection. The droplets will cause large noise spikes that obscure the spectrum. This problem becomes greater at larger liquid flow rates into the electrospray. Prior art devices have used one of the focusing electrodes as a gate to stop ions from entering the trap during the scanning process by applying a large repulsive potential to stop the ions from passing through the lens. See U.S. Pat. No. 5,750,993 for example. However, this requires the charged droplets to stop and reverse direction to avoid entering the end cap hole which creates background noise in the detector.
The present invention is directed to reducing background noise from charged droplets.